High speed automatic identification of random size items has historically been attempted using various optical, magnetic and RF means. By far the most popular is the bar code label. Bar codes are typically read using a low speed (&lt;1000 scans per second) laser line scanner. Normally the laser line must be aligned with the axis of the bar code in such a manner that a single laser line passes through all the bars of the code in a single pass. Although the bar code inherently has a large amount of vertical redundancy to allow for error correction by multiple passes through the data, it is possible for a label to be damaged in such a way as to render no path through a completely undamaged position of the label even though the total damage to the label is small. The large amount of vertical redundancy is also the main drawback of the bar code, because it limits the density of the bar code, i.e., the bar code has a practical limit of around 30 ASCII characters.
Omnidirectional bar code readers have recently been introduced that do not require a complete pass through an entire label by a single line to read the bar code information. These omnidirectional bar code readers acquire positions of a bar code label on successive passes through the label and then typically use a high performance processor to piece these fragments of a label together by looking for overlap between the fragments and/or timing the movement of the item bearing the label with respect to the laser line. To achieve true omnidirectional capability and to cover a large field of view, a typical omnidirectional scanner uses multiple laser beams placed at various angles and locations.
Many other optical data encoding techniques have been proposed. Stacked bar codes and area codes are the most promising of these. These codes have not been widely adopted because a practical reader for them has not been produced. Raster laser line scanners are available for reading stacked bar codes, but these typically have limited fields of view, limited depths of field and are not high speed or omnidirectional. They are only practical in human interactive applications where the operator is required to provide rough alignment between the reading device and the label.
The area or line CCD camera can be the basis for an omnidirectional bar code or area code reading device. The camera is used to assemble an image of the scene in a frame buffer and then this image is processed using standard image processing techniques to find, orient and decode the bar code or area code label. However, the CCD camera has many drawbacks as the image collection device for a reader. Limitations of the area CCD camera as a sensor include field of view (FOV) versus cost, depth of field (DOF), and lighting requirements. The line CCD camera has the same DOF and lighting problems.
The field of view of a cost effective omnidirectional bar code or area code reader using an area CCD camera is limited by the cost of the CCD semiconductor sensor. To read a code with a 30 mil X dimension requires approximately 10 mil samples (i.e., three pixel per min discernible feature). A typical 512.times.512 CCD camera can cover a maximum field of view 5 inches square. While this may be useful for some applications (pharmaceutical package verification for example) the ability to scale this to a large area like a 30 inch (or 1 meter) wide conveyor is limited by the rapid increase in cost of the CCD sensor. This cost is due to the physical size of the semiconductor which limits the number of parts that fit on a wafer and the number of elements in the device which limits the yield of the semiconductor process. Because of these factors and the limited demand for large area CCD sensors, the cost is not expected to drop appreciably in the near future.
One alternative is to use two sensors, a low resolution sensor to cover a large FOV and identify possible areas of interest coupled with a high resolution sensor with a mechanical aiming device to focus on a small FOV which contains a label. This system is limited by the speed and complexity of the mechanical mechanism and cannot process an image cluttered with many areas of interest typical of a package sorting application.
The CID array has been investigated as a solid state alternative to the two camera high/low resolution system. The CID array differs from a conventional CCD array in that pixels can be addressed and read randomly and non-destructively. This would allow it to be read out first at low resolution to identify areas of interest and then the areas containing possible labels re-read in high resolution to identify and decode the actual labels. A practical CID array does not currently exist. It is also likely that a large CID sensor would have the same high piece cost as CCD sensors.
Typical cost effective CCD cameras are limited to a two to four inch depth of field without being refocused. Better lenses and faster shutters coupled with brighter lighting can expand the DOF. However, due to the specialty nature of these lenses, the cost rises rapidly with DOF improvements. A mechanical refocusing system can provide a large DOF; however, it requires an additional height sensor and a significant time to move the lenses. There are cameras available which can focus by moving the sensor rather than the lenses. The low mass of the sensor allows this system to be very fast; however, it is an expensive option.
To image a moving object with a CCD camera requires the image to be electronically or mechanically "stopped" by reducing the duration of exposure of the sensor to less than the time for the object to move enough to smear the image. The faster the object is moving the shorter the exposure duration must be to "stop" the motion of the object. To achieve sensitivity at fast shutter speeds requires progressively brighter lighting. A strobe (or flash) light is the normal method of producing this light. A strobe may not be acceptable for use in an industrial environment around moving machinery because of the safety issues involved. Lighting of acceptable intensity, lifetime and compliance with an industrial environment can be a costly additional component in a reader.